This was not implementing the following part of the spec correctly:
27. For each integer i such that i ≥ 1 and i ≤ n, do
a. Let captureI be ith element of r's captures List.
b. If captureI is undefined, let capturedValue be undefined.
Expecting a capture group match to exist for each of the RegExp's
capture groups would assert in Vector's operator[] if that's not the
case, for example:
/(foo)(bar)?/.exec("foo")
Append undefined instead.
Fixes#5256.
